1. Field of the Invention
The present invention generally relates to an electric rotating machine incorporated in a starter for starting an engine. Particularly, the present invention relates to a yoke for electric rotating machine incorporated for starter motors, etc., which are used under high vibration conditions in vehicles, etc., and have permanent magnets used for a field magnetic pole.
2. Related Art
In recent years, starters using permanent magnets instead of the conventional winding type stator have become commonly used for electric rotating machine in order to reduce the weight of the electric rotating machine and to reduce production costs, etc. The configuration and assembly configuration of a yoke of this type of electric rotating machine are shown in FIGS. 5A, 5B, 6A and 6B. The yoke unit 50 acts as the outer shell of an electric rotating machine and is configured of a tubular yoke 11, and lid portions 14 which cover both axial ends of the tubular yoke 11. In the electric rotating machine using permanent magnets for the stator magnetic pole, multiple permanent magnets 12 are set in the inner circumference of the tubular yoke 11. The lid portions 14 set on both axial ends of tubular yoke 11 in the electric rotating machine are, as illustrated in FIG. 7, formed to be larger than the outer diameter of each tubular yoke 11 by the amount of the diameter of the through bolt explained later. The lid portions 14 on both ends of the tubular yoke 11, are tightened and fixed to,both ends of the tubular yoke 11 by the multiple through bolts 13 at a flange portion 15 that protrudes by the amount of the approximate diameter of the through bolts 13 onto the outer said of the outer diameter of the tubular yoke 11. In the electric rotating machine illustrated in FIG. 7, the diameter of the lid portion 14 is formed to approximately the same as the outer diameter of the tubular yoke 11. The lid portions 14 on both ends of the tubular yoke 11 are tightened and fixed to both ends of the tubular yoke 11 by the through bolts 13 that pass through the tubular yoke 11.
High performance ferrite magnets, etc., which have a cost lower than winding type stators are used for the permanent magnets 12 incorporated for the electric rotating machine illustrated in FIGS. 5A through 6B. These ferrite magnets have a residual flux density that is relatively low at 3000 G (gauss) to 5000 G, and thus the flux amount is small. Thus, when using these types of permanent magnets 12 for the stator of the electric rotating machine, the metallic tubular yoke 11 that congregates the field can be thin as compared to when a winding type stator is used. As the tubular yoke 11 can be thinner, the weight and cost of the electric rotating machine can be reduced.
However, in the case that this type of electric rotating machine is incorporated for applications such as vehicle starter motors and used under high vibration conditions, if the tubular yoke 11 which is thin as described above is used, the rigidity is low and the yoke unit 50 will resonate with the external vibration from the engine, etc. This can lead to damage of each electric rotating machine part such as loosening in the tightening and fixing with the through bolts 13, or can lead to abnormal wear in each part, thereby affecting the life of the electric rotating machine or starter. To prevent this type of disadvantages, the thickness of the tubular yoke 11 can be increased, or the number of through bolts 13 for tightening and fixing the lid portions 14 can be increased for example, but this leads to problems when trying to reduce weight or costs, etc. of the electric rotating machine.
Furthermore, in the above case, if the tubular yoke 11 and lid portions 14 are tightened and fixed to the inner circumference of the tubular yoke 11 at the flange portion 15 or lid portion 14 with through bolts 13 as shown in FIGS. 6A and 6B, a bend moment is applied to the lid portion 14 centering on the end portion of the tubular yoke 11. At this time, if the tightening force of the through bolts 13 is increased, the lid portion 14 may deform. For example, in the electric rotating machine illustrated in FIGS. 5A and 5B, a bend moment will be applied in the direction .alpha.1 as shown in FIG. 7, and the lid portion 14 will deform in the direction .alpha.2. In the yoke for electric rotating machine illustrated in FIGS. 6A and 6B, a bend moment will be applied in the direction .beta.1 as shown in FIG. 8, and the lid portion 14 will deform in the direction .beta.2. If the rigidity of the lid portion 14 is increased by using a material with a high rigidity for the lid portion 14, etc., the tubular yoke 11 formed by a thin steel plate, etc., with a low rigidity may be strained. Using material with a high rigidity or increasing the thickness to improve the rigidity of the lid portion 14 as described above will lead to problems When trying to reduce weight or costs, etc. of the electric rotating machine.
It has been difficult to secure the rigidity of the tubular yoke 11 and to sufficiently utilize the tightening force of the through bolts 13 when assembling onto the yoke 11 for the electric rotating machine that incorporates a thin steel plate tubular yoke for reducing weight and costs.